In general,
a window should never set its own position; this should be done only by the geometry manger that is responsible for the window.
For top-level windows the window manager is effectively the geometry manager; Tk provides interface code between the application and the window manager to convey the application's desires to the geometry manager.
The desired size for a top-level window is conveyed using the usual Tk_GeometryRequest mechanism.
The procedure Tk_MoveToplevelWindow may be used by an application to request a particular position for a top-level window; this procedure is similar in function to the wm geometry Tcl command except that negative offsets cannot be specified.
It is invoked by widgets such as menus that want to appear at a particular place on the screen.

When Tk_MoveToplevelWindow is called it doesn't immediately pass on the new desired location to the window manager; it defers this action until all other outstanding work has been completed,
using the Tk_DoWhenIdle mechanism.